This invention relates generally to testing and calibration of electroacoustic transducers, and more particularly to a novel method and apparatus for determining relative phase and sensitivity characteristics of elements of a long multi-element, high frequency array for the purpose of determining phase trimming and sensitivity shading necessary to achieve accurate near field focusing of the array.
In an ideal situation, measurements of phase and sensitivity parameters of the array receiving elements would be made while the elements of the array are simultaneously subjected to incidence of a high frequency, plane wavefront. Conventional methods have used a substantially plane wave produced by placing projectors in the far field region, by using collimators for converting a curved wave front to plane, or by using a projecting array having a plurality of projectors shaded so as to generate a substantially plane wave. The last method is described in U.S. Pat. No. 3,393,400 to W. J. Trott, assigned to the assignee hereof.
These prior art techniques, while of considerable use in certain situations, are subject to disadvantages, particularly when operating at frequencies in the megaherz ranges. Thus, for long, high frequency arrays the required distance between a source projector and the array is extremely large and creates many difficult problems of alignments and compensations for environmental constraints. Acoustic collimators and calibrating arrays of the type mentioned in the above noted patent are expensive and alignment is extremely difficult.